Orthogonal Frequency-Division Multiple Access (OFDMA) is a multiple access method for sharing a radio frequency (RF) channel among multiple stations. OFDMA uses an orthogonal frequency-division multiplexing (OFDM) digital modulation scheme to modulate information signals. OFDMA can be described as a combination of frequency domain and time domain multiple access. In OFDMA, a communication space is divided into multiple timeslots and each timeslot is further divided into a number of frequency sub-channels, each having at least one of its own sub-carriers. In OFDMA systems, both time and/or frequency resources are used to separate signals to/from multiple stations, wherein transmissions to/from multiple stations are separated using timeslots and sub-channels within each timeslot such that stations' signals can be separated in the time domain and/or in the frequency domain. Thus, in OFDMA, resources can be partitioned in the time-frequency space.
Recently, broadband wireless networks have been developed that implement OFDMA. The Institute of Electrical and Electronics Engineers (IEEE) 802.16 networks are one example. As used herein, “IEEE 802.16” refers to a set of IEEE Wireless Metropolitan Area Network (WMAN) standards that govern broadband wireless access methods. Any of the IEEE standards or specifications referred to herein may be obtained at IEEE, 445 Hoes Lane, PO Box 1331, Piscataway, N.J. 08855-1331, USA. In addition, a Long Term Evolution (LTE) project has been created to provide for a high speed wireless data communications network. LTE is a Third Generation Partnership Project (3GPP) project developed by the European Telecommunications Standards Institute (ETSI). In a LTE network, an evolved Node B (eNB) is a mandatory node in a radio access network (RAN) portion of the LTE network. Similar to a base station in an 802.16 WMAN, the eNB in the LTE network handles radio communications with multiple devices in a cell and carries out radio resource management and handover decisions. Similar to the subscriber station (SS) in an 802.16 WMAN, the user equipment (UE) in the LTE network serves as a device that communicates with the eNB. The user equipment and/or the subscriber station are referred to in this document as a station. Any of the ETSI standards or specifications referred to herein may be obtained at 650, Route des Lucioles, 06921 Sophia-Antipolis Cedex, FRANCE.
In a wireless communication system, a near-far problem may exist. The near-far problem refers to the situation where a receiving station receives a low-power signal from a desired transmitting station and a high-power signal from a different transmitting station at the same time, resulting in desensitization or “desense” of a receiver in the receiving station to the low-power signal. In other words, the high-power signal may cause the low-power signal to fall below the receiver's detectability threshold. For instance, when the high power transmitting station is located near the receiving station operating in the same timeslot but on a different frequency sub-channel, the high transmit energy can desensitize the receiver.
In a wireless communication system, duplex operation is supported whereby an eNB and a station can both transmit and receive information. With a half-duplex operation, a station cannot transmit and receive information at the same time. With a full duplex operation, a station can transmit and receive information at the same time. Half-duplex operation can be realized with time division duplex (TDD) where a single frequency channel is divided in time with a portion of the time reserved for transmission from an eNB to a station, commonly referred to as the downlink, and a separate orthogonal portion of the time reserved for transmission from a station to an eNB, commonly referred to as the uplink. Full-duplex operation can be realized with frequency division duplex (FDD) where two frequency channels are utilized. One frequency channel is dedicated for downlink transmissions from an eNB to a station while the second simultaneously operating frequency channel is dedicated for uplink transmissions from a station to an eNB. Both the TDD and FDD frequency channels may be divided into timeslots in support of OFDMA.
Scheduling algorithms are widely used in wireless networks for allocating or distributing communication resources (e.g., timeslots and/or sub-channels) among stations to take advantage of instantaneous channel variations by giving priority to the stations with favorable channel conditions. For instance, in an OFDMA communication system, a base station or eNB can include a time-division multiple access (TDMA) scheduler that schedules time/frequency resources used by each uplink communication and each downlink communication. More than one uplink communication may originate from a transmitter of one or more stations whereas the downlink communication originating from the transmitter of a base station or eNB is intended for receiver(s) of one or more stations. The scheduler may assign an uplink communication on different sub-channels within the same timeslot to different stations. In particular, the base station or eNB scheduler may schedule these uplink communications either in different timeslots or in the same timeslot and uses power control to prevent/reduce near-far interference among various stations communicating to the eNB. Accordingly, the TDMA scheduler avoids near-far problems by either creating time-orthogonal uplink transmissions or through uplink power control. The scheduler may assign a downlink communication on different sub-channels within the same timeslot from an eNB to different stations. The near-far interference issue is avoided in that there is only the single desired transmitter and no interfering transmitter operating simultaneously. These techniques are applicable to TDD wireless communication systems where a select orthogonal portion of time has been set aside within the frequency channel for uplink and downlink transmissions and FDD wireless communication systems where a separate frequency channel is dedicated to uplink and downlink transmissions.
Although the TDMA scheduling techniques described above work well in situations where all stations communicate with and are assigned or scheduled resources by a central base station or eNB, these techniques do not work in mixed networks that also include direct station-to-station or “peer-to-peer” communication between stations. When one station connects directly with and communicates directly with another station, this method of communication is referred to herein as peer-to-peer communication. With peer-to-peer communication, there is no concept of uplink and downlink because communications links occur between the stations and do not necessarily involve the eNB. When peer-to-peer communication links are allowed to share a portion of the time-frequency resources whether within conventional uplink or downlink resource allocations or resource allocations occupied by peer-to-peer communication links alone, near-far interference can occur. The near-far interference can desense either the eNB-to-station and station-to-eNB communication links or the peer-to-peer links.
Peer-to-peer communication can be directed by a base station or eNB (centralized scheduling), or it can be self-directed by stations involved in the communication (distributed scheduling). With centralized scheduling, stations communicate with the base station or eNB via control channels to exchange information needed for scheduling. With distributed scheduling stations communicate with each other in order to exchange information needed to schedule resources. Ad-hoc mesh network may expand the communications range of peer-to-peer networks, wherein mesh nodes or stations could collect and forward routing, neighbor list and other information either to a centralized base station/eNB scheduler or to stations employing distributed scheduling. OFDMA communication systems allowing peer-to-peer communications employing either centralized scheduling or distributed scheduling with or without mesh ad-hoc may experience significant near-far interference when conventional TDMA uplink/downlink scheduling is being used
Accordingly, it would be desirable to provide scheduling techniques for scheduling peer-to-peer communication links.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.